Ranging from teak trees to bamboo, they contain some of the most iconic species of endangered mammals in the world.

"This includes things like tigers, leopards, elephants, bears — all of these species of huge global significance," said Clare Campbell, executive director of Karen Wildlife Conservation Initiative (KWCI).

"I think it's probably one of the most significant [conservation] areas certainly in Asia, if not in the world."

The Perth-based group has coordinated the first major survey of the area using remote sensor cameras.

The images of big cats, some with cubs, has thrilled conservationists.

"With leopards it's particularly significant because we know there's a breeding population — there's a solid population of healthy leopards in here and a full prey base to support them," Ms Campbell says.

A scientific paper about the survey, published this month, said Karen state, in Myanmar, held "one of the most significant leopard populations remaining in South-East Asia".

For tigers, the forest's location on the border with Thailand is crucial.

"The Thai population of tigers in the western forest complex was probably the last significant population of tigers in this whole region ... but these two populations are moving between the two regions," says Ms Campbell.

The survey area is controlled by the Karen National Union, an ethnic group that has fought the government for decades over autonomy, but signed a ceasefire in 2015.

Fighting has meant the region has been largely off-limits to researchers, until now.

With funding and ground support from a range of organisations, the group was able to record 10,000 days and nights of animal appearances.

Images of poachers cause for concern

"Sadly the reality is there is hunting starting to increase in some of these areas for the illegal wildlife trade ... which is just further reason why we need to increase the direct protection on the ground," Ms Campbell says.

Direct protection means rangers.

Already six 10-man teams patrol the forests, but that's not enough.

"Urgent action is thus needed to assist the Karen people to protect one of South-East Asia's last intact rich and diverse ecosystems," write co-authors Saw Sha Bwe Moo, Graden Froese and Thomas Gray in the Oryx Journal.

One thing going for the Karen forests is the attitude of the local population, with cultural taboos forbidding the hunting of key species, like the great hornbill and tigers.

"The tigers kind of represent the spirit of the forest ... and the hornbill are like Karen couples in their past life," says Paul Sein Twa, director of the Karen Environmental and Social Action Network.

In so many places that "spirit" animal has gone.

Ms Campbell says hunting and logging in some parts of Asia has left what's known as "empty forest syndrome", where you can stand amongst trees but hear no birdcalls.